Mufflers are used, for example, as components of exhaust systems of motor vehicles in general, and in particular as components of exhausts systems of cars. The muffler is designed to reduce the noise levels generated by the internal combustion engine. To this end, the acoustical waves propagate through a series of resonating chambers tuned to attenuate the noise. Alternatively or in addition, fibrous material may be arranged in one or more chambers of the muffler to attenuate the loudness of the sound pressure. The chambers within the muffler are formed by sectional plates called baffles which are inserted into a shell during assembly of the muffler. The baffles additionally may support various pipes within the muffler, for example the inlet pipe and the outlet pipe of the muffler.
Generally, a chamber of the muffler is bounded by a pair of baffles or between baffle and endcap and the shell. A first pipe may be arranged to extend through the baffle plate of one baffle of the pair and may have a discharge end opening out into the chamber. A second pipe may extend through the baffle plate of the other baffle of the pair into the chamber and may have an inlet end also opening out into the chamber for allowing exhaust gas and acoustic waves to exit from the chamber. In some embodiments of mufflers, a perforated flow pipe may extend completely through the chamber bounded by the two baffles and the shell, allowing exhaust gas and acoustic waves to enter and exit the chamber through these perforations. In still further embodiments of mufflers, the flow pipe is not perforated and constitutes a bypass of the chamber, so that exhaust gas and acoustic waves may enter and exit the bypassed chamber either through discharge and inlet ends of other pipes opening out into this chamber, or through perforations provided in the baffle plates bounding this chamber.
During assembly of the muffler, the baffles are inserted into the shell by a process called “stuffing”. The baffles not only comprise the baffle plate but in addition comprise a bent edge called “baffle lip” which upon being stuffed into the shell is in a frictional sliding contact with the shell.
As is evident, the baffles have different functions. First, the baffles divide the muffler into acoustical compartments. The baffles may additionally contain holes which create an acoustical connection between the acoustical compartments. Second, the baffles support the pipes. Third, the baffles to some extent decouple the elements arranged in the interior of the muffler and the shell with respect to thermal expansion by providing a sliding contact between the interior elements of the muffler and its housing.
During stuffing in the assembly of the muffler as well as upon thermal expansion or contraction in operation, frictional forces are exerted on the baffle lip. These frictional forces may increase due to “baffle edge gripping” (the edge of the baffle “grips” into the inner wall of the shell). Baffle edge gripping may lead to different sliding properties of the baffle edge in the two opposite directions along the inner wall of the shell and is a consequence of the process of manufacturing the baffles (including stamping and cutting of metal sheets which results in spring-back of the tips of the baffle lip). This process of manufacturing the baffles lead to baffle lip tips which are bent outwardly towards the inner wall of the muffler shell.
The afore-described outwardly bent baffle lip tips also have a negative impact on the “stick and slip” movement of the baffle. The stick and slip movement of the baffle typically occurs during the heat-up of the muffler during operation, or the cooling down of the muffler after operation and generates the well-known “tick and ping” noise.
The frictional forces may result in bending of the baffle plate. Accordingly, reduction of the wall thickness of the baffle (and an associated saving of weight) is very limited only, since a further reduction of the wall thickness of the baffle would increase the tendency of the baffle plate to bend due to reduced stiffness of the baffle plate which may create problems regarding the stuffing of the baffle plate into the muffler shell during assembly of the muffler and which may also create loss of the function of the muffler due to thermal cycling.